Power storage device, lithium-ion secondary battery, electric double layer capacitor and lithium-ion capacitor

ABSTRACT

One object is to provide a power storage device including an electrolyte using a room-temperature ionic liquid which includes a univalent anion and a cyclic quaternary ammonium cation having excellent reduction resistance. Another object is to provide a high-performance power storage device. A room-temperature ionic liquid which includes a cyclic quaternary ammonium cation represented by a general formula (G1) below is used for an electrolyte of a power storage device. In the general formula (G1), one or two of R 1  to R 5  are any of an alkyl group having 1 to 20 carbon atoms, a methoxy group, a methoxymethyl group, and a methoxyethyl group. The other three or four of R 1  to R 5  are hydrogen atoms. A −  is a univalent imide anion, a univalent methide anion, a perfluoroalkyl sulfonic acid anion, tetrafluoroborate (BF 4   − ), or hexafluorophosphate (PF 6   − ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a room-temperature ionic liquid and apower storage device using the room-temperature ionic liquid.

Note that the power storage device indicates all elements and deviceswhich have a function of storing power.

2. Description of the Related Art

A lithium-ion secondary battery which is one of power storage devices isused in a variety of applications including mobile phones, electricvehicles (EV), and the like. Characteristics such as high energydensity, cycle characteristics, and safety under various operatingenvironments are required for a lithium-ion secondary battery.

As an organic solvent for an electrolyte of a lithium-ion secondarybattery, a cyclic carbonate which has high dielectric constant andexcellent ion conductivity is often used. Among the cyclic carbonate,ethylene carbonate is often used.

However, not only ethylene carbonate but many organic solvents havevolatility and a low flash point. For this reason, in the case where anorganic solvent is used for an electrolyte of a lithium-ion secondarybattery, the temperature inside the lithium-ion secondary battery mightrise due to a short circuit, overcharge, or the like and the lithium-ionsecondary battery might burst or catch fire.

In view of the above, it has been considered to use a room-temperatureionic liquid which is less likely to burn and volatilize as anelectrolyte of a lithium-ion secondary battery.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2003-331918

SUMMARY OF THE INVENTION

When a room-temperature ionic liquid is used for an electrolyte of alithium-ion secondary battery, there is a problem in that a lowpotential negative electrode material cannot be used because of lowreduction resistance of a room-temperature ionic liquid. Thus, atechnique has been disclosed, which enables dissolution andprecipitation of lithium which is a low potential negative electrodematerial without an additive by improving the reduction resistance of aroom-temperature ionic liquid using quaternary ammonium salt (see PatentDocument 1). However, the reduction potential of a room-temperatureionic liquid whose reduction resistance is thus improved issubstantially equivalent to an oxidation-reduction potential of lithium.Further improvement is required for the reduction resistance of aroom-temperature ionic liquid.

In view of the above problem, one object of the present invention is toprovide a power storage device including an electrolyte using aroom-temperature ionic liquid which is excellent in reductionresistance. Another object is to provide a high-performance powerstorage device.

One embodiment of the present invention is a power storage deviceincluding a room-temperature ionic liquid represented by a generalformula (G1) below, which includes a cyclic quaternary ammonium cation.

In the general formula (G1), one or two of R₁ to R₅ are any of an alkylgroup having 1 to 20 carbon atoms, a methoxy group, a methoxymethylgroup, and a methoxyethyl group; the other three or four of R₁ to R₅ arehydrogen atoms; and A⁻ is a univalent imide anion, a univalent methideanion, a perfluoroalkyl sulfonic acid anion, tetrafluoroborate (BF₄ ⁻),or hexafluorophosphate (PF₆ ⁻).

Specifically, one or two of R₁ to R₅ in the room-temperature ionicliquid represented by the general formula (G1) are preferably an alkylgroup having 1 to 4 carbon atoms.

Another embodiment of the present invention is a power storage deviceincluding an electrolyte using a room-temperature ionic liquidrepresented by a general formula (G2) below, which includes a cyclicquaternary ammonium cation.

In the general formula (G2), one of R₁ and R₂ is any of an alkyl grouphaving 1 to 20 carbon atoms, a methoxy group, a methoxymethyl group, anda methoxyethyl group; the other of R₁ and R₂ is a hydrogen atom; and A⁻is a univalent imide anion, a univalent methide anion, a perfluoroalkylsulfonic acid anion, tetrafluoroborate (BF₄ ⁻) or hexafluorophosphate(PF₆ ⁻).

Another embodiment of the present invention is a power storage deviceincluding an electrolyte using the room-temperature ionic liquidrepresented by the general formula (G2), in which one of R₁ and R₂ is analkyl group having 1 to 4 carbon atoms.

Another embodiment of the present invention is a power storage deviceincluding an electrolyte using a room-temperature ionic liquidrepresented by a general formula (G3) below, which includes a cyclicquaternary ammonium cation.

In the general formula (G3), A⁻ is a univalent imide anion, a univalentmethide anion, a perfluoroalkyl sulfonic acid anion, tetrafluoroborate(BF₄ ⁻), or hexafluorophosphate (PF₆ ⁻).

Another embodiment of the present invention is a power storage deviceincluding an electrolyte using a room-temperature ionic liquidrepresented by a general formula (G4) below, which includes a cyclicquaternary ammonium cation.

In the general formula (G4), A⁻ is a univalent imide anion, a univalentmethide anion, a perfluoroalkyl sulfonic acid anion, tetrafluoroborate(BF₄ ⁻), or hexafluorophosphate (PF₆ ⁻).

Another embodiment of the present invention is a power storage deviceincluding an electrolyte using a room-temperature ionic liquid whichincludes any one of univalent anions selected from (C_(n)F_(2n+1)SO₂)₂N⁻(n=0 to 4), (C_(m)F_(2m+1)SO₃)⁻ (m=0 to 4), and CF₂(CF₂SO₂)₂N⁻ for A⁻ inthe general formulae (G1) to (G4).

Another embodiment of the present invention is a lithium-ion secondarybattery at least including a positive electrode, a negative electrode,any one of the room-temperature ionic liquids represented by the generalformulae (G1) to (G4), and electrolyte salt including lithium. Any oneof the room-temperature ionic liquids represented by the generalformulae (G1) to (G4) and the electrolyte salt including lithium areincluded in an electrolyte.

Another embodiment of the present invention is an electric double layercapacitor at least including a positive electrode, a negative electrode,and any one of the room-temperature ionic liquids represented by thegeneral formulae (G1) to (G4) which is used for an electrolyte.

Another embodiment of the present invention is a lithium-ion capacitorat least including a positive electrode, a negative electrode, any oneof the room-temperature ionic liquids represented by the generalformulae (G1) to (G4), and electrolyte salt including lithium. Any oneof the room-temperature ionic liquids represented by the generalformulae (G1) to (G4) and the electrolyte salt including lithium areincluded in an electrolyte.

According to one embodiment of the present invention, a power storagedevice including an electrolyte using a room-temperature ionic liquidwhich is excellent in reduction resistance can be provided. Further, ahigh-performance power storage device can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are cross-sectional views of lithium-ion secondarybatteries.

FIG. 2A is a top view of a lithium-ion secondary battery and FIG. 2B isa perspective view of a lithium-ion secondary battery.

FIGS. 3A and 3B are perspective views showing a manufacturing method ofa lithium-ion secondary battery.

FIG. 4 is a perspective view showing a manufacturing method of thelithium-ion secondary battery.

FIG. 5 is a perspective view showing an example of an application modeof a power storage device.

FIGS. 6A and 6B are graphs each showing NMR charts of1,2-dimethyl-1-propylpiperidinium bis(trifluoromethanesulfonyl)imide.

FIGS. 7A and 7B are graphs each showing NMR charts of1,3-dimethyl-1-propylpiperidinium bis(trifluoromethanesulfonyl)imide.

FIG. 8 is a graph showing linear sweep voltammograms of synthesizedsamples and a comparative sample.

FIGS. 9A and 9B are graphs each showing NMR charts of1,3-dimethyl-1-propylpiperidinium bis(fluorosulfonyl)imide.

FIG. 10 is a perspective view showing a manufacturing method of alithium-ion secondary battery.

FIG. 11 is a graph showing charge and discharge characteristics of amanufactured lithium-ion secondary battery.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter embodiments of the present invention will be described indetail with reference to the accompanying drawings. Note that thepresent invention is not limited to the following description and it iseasily understood by those skilled in the art that the mode and detailscan be variously changed without departing from the scope and spirit ofthe present invention. Therefore the invention should not be construedas being limited to the description of the embodiment below. Indescribing structures of the present invention with reference to thedrawings, the same reference numerals are used in common for the sameportions in different drawings. The same hatching pattern is applied tosimilar parts, and the similar parts are not especially denoted byreference numerals in some cases. In addition, an insulating layer isnot illustrated in a top view for convenience in some cases. Note thatthe size, the layer thickness, or the region of each structure shown ineach drawing is exaggerated for clarity in some cases. Consequently, thepresent invention is not necessarily limited to such scales shown in thedrawings.

Embodiment 1

In this embodiment, an electrolyte of a power storage device which isone embodiment of the present invention, and a room-temperature ionicliquid which is used for the electrolyte, which is one embodiment of thepresent invention, will be described.

A room-temperature ionic liquid of one embodiment of the presentinvention includes a cyclic quaternary ammonium cation and a univalentanion. The room-temperature ionic liquid can be represented by a generalformula (G1) below.

In the general formula (G1), one or two of R₁ to R₅ are any of an alkylgroup having 1 to 20 carbon atoms, a methoxy group, a methoxymethylgroup, and a methoxyethyl group. The other three or four of R₁ to R₅ arehydrogen atoms. A⁻ is a univalent imide anion, a univalent methideanion, a perfluoroalkyl sulfonic acid anion, tetrafluoroborate (BF₄ ⁻),or hexafluorophosphate (PF₆ ⁻).

In the case where R₁ to R₅ in the general formula (G1) are an alkylgroup having 1 to 20 carbon atoms, carbon atoms having small carbonnumber (for example, 1 to 4 carbon atoms) is used because the viscosityof the synthesized room-temperature ionic liquid can be reduced; whichis preferable for a power storage device.

When a room-temperature ionic liquid having low reduction resistance(also referred to as stability against reduction) is used for anelectrolyte of a power storage device, the room-temperature ionic liquidis reduced by receiving electrons from a positive electrode material ora negative electrode material and therefore decomposed. Characteristicsof the power storage device deteriorate as a result.

“Reduction of a room-temperature ionic liquid” means that aroom-temperature ionic liquid receives electrons from a positiveelectrode material or a negative electrode material. Thus, the stabilityagainst reduction can be improved by making it difficult particularlyfor a cation having a positive charge, which is included in theroom-temperature ionic liquid, to receive electrons, i.e., lowering thereduction potential of the room-temperature ionic liquid.

Inductive effects are caused by an electron donating substituentincluded in the room-temperature ionic liquids of embodiments of thepresent invention. In the room-temperature ionic liquid, electricpolarization of a cation which is an ion having a positive charge isalleviated due to inductive effects, so that it is difficult for thecation to receive electrons. Consequently, the reduction resistance ofthe room-temperature ionic liquid is improved.

As described above, the electron donating substituent can be any one ofan alkyl group having 1 to 20 carbon atoms, a methoxy group, amethoxymethyl group, and a methoxyethyl group. The alkyl group having 1to 20 carbon atoms may be either a straight-chain alkyl group or abranched-chain alkyl group.

The reduction potential of the room-temperature ionic liquid can belowered and the reduction resistance of the room-temperature ionicliquid can be improved even when the general formula (G1) includeseither one or two electron donating substituents.

A room-temperature ionic liquid of one embodiment of the presentinvention includes a cyclic quaternary ammonium cation and a univalentanion. The room-temperature ionic liquid can be represented by a generalformula (G2) below.

One of R₁ and R₂ in the general formula (G2) is any one of an alkylgroup having 1 to 20 carbon atoms, a methoxy group, a methoxymethylgroup, and a methoxyethyl group. The other of R₁ and R₂ is a hydrogenatom. A⁻ is a univalent imide anion, a univalent methide anion, aperfluoroalkyl sulfonic acid anion, BF₄ ⁻, or PF₆ ⁻.

It is preferable to use a carbon atom having small carbon number for analkyl group in the general formula (G2) as in the general formula (G1)because a cyclic quaternary ammonium cation can be easily synthesized.

In the general formula (G2), R₃ to R₅ of the general formula (G1) arehydrogen atoms, so that the general formula (G2) is a room-temperatureionic liquid whose reduction resistance is improved.

Further, one embodiment of the present invention is a room-temperatureionic liquid including a methyl group as R₁ and a hydrogen atom as R₂ inthe general formula (G2). The room-temperature ionic liquid can berepresented by a general formula (G3) below. AT is any one of aunivalent imide anion, a univalent methide anion, a perfluoroalkylsulfonic acid anion, BF₄ ⁻, and PF₆ ⁻.

Furthermore, one embodiment of the present invention is aroom-temperature ionic liquid including a hydrogen atom as R₁ and amethyl group as R₂ in the general formula (G2). The room-temperatureionic liquid can be represented by a general formula (G4) below. A⁻ isany one of a univalent imide anion, a univalent methide anion, aperfluoroalkyl sulfonic acid anion, BF₄ ⁻, and PF₆ ⁻.

The general formulae (G3) and (G4) are room-temperature ionic liquidsbased on the general formula (G2) and further based on the generalformula (G1). Therefore, the general formulae (G3) and (G4) areroom-temperature ionic liquids whose reduction resistance is improved.

Further, A⁻ in the general formulae (G1) to (G4) is any one of aunivalent imide anion, a univalent methide anion, a perfluoroalkylsulfonic acid anion, BF₄ ⁻, and PF₆ ⁻; however, A⁻ is not limited tothis. Any anion may be used as A⁻ as long as it serves as aroom-temperature ionic liquid with a cyclic quaternary ammonium cationof one embodiment of the present invention.

Here, calculation results of an improvement of the reduction resistancecaused by an electron donating substituent are shown.

A lowest unoccupied molecular orbital level (LUMO level) of a cation ineach of six kinds of room-temperature ionic liquids determined by aquantum chemistry computational program is shown in Table 1. The sixkinds of room-temperature ionic liquids each include a methyl group assubstituents of R₁ to R₅ in the general formula (G1). The six kinds ofroom-temperature ionic liquids are represented by structural formulae(α-1) to (α-8) below. In addition, as a comparative example, a lowestunoccupied molecular orbital level (LUMO level) of a(N-methyl-N-propylpiperidinium) cation represented by a structuralformula (α-9) below is shown in the Table 1. A(N-methyl-N-propylpiperidinium) cation is a room-temperature ionicliquid having a reduction potential which is the same degree as that ofan oxidation-reduction potential of lithium which is used as a negativeelectrode of a power storage device.

TABLE 1 LUMO Level Structural Formula (α-1) −3.047 [eV] StructuralFormula (α-2) −3.174 [eV] Structural Formula (α-3) −3.192 [eV]Structural Formula (α-4) −2.941 [eV] Structural Formula (α-5) −3.013[eV] Structural Formula (α-6) −2.877 [eV] Structural Formula (α-7)−3.125 [eV] Structural Formula (α-8) −3.102 [eV] Structural Formula(α-9) −3.244 [eV]

In the quantum chemistry computation of this embodiment, the optimalmolecular structures in the ground state and a triplet state of a cationin each of the room-temperature ionic liquids of embodiments of thepresent invention and a (N-methyl-N-propylpiperidinium) cation arecalculated by using the density functional theory (DFT). The totalenergy of the DFT is represented as the sum of potential energy,electrostatic energy between electrons, electronic kinetic energy, andexchange-correlation energy including all the complicated interactionsbetween electrons. Also in the DFT, an exchange-correlation interactionis approximated by a functional (a function of another function) of oneelectron potential represented in terms of electron density to enablehigh-speed and highly-accurate calculations. Here, B3LYP, which is ahybrid functional, is used to specify the weight of each parameterrelated to exchange-correlation energy. In addition, as a basisfunction, 6-311 (a basis function of a triple-split valence basis setusing three contraction functions for each valence orbital) is appliedto all the atoms. By the above basis function, for example, orbits of 1sto 3s are considered in the case of hydrogen atoms while orbits of 1s to4s and 2p to 4p are considered in the case of carbon atoms. Furthermore,to improve calculation accuracy, the p function and the d function aspolarization basis sets are added to hydrogen atoms and atoms other thanhydrogen atoms respectively.

Note that Gaussian 09 is used as the quantum chemistry computationalprogram. A high performance computer (Altix 4700, manufactured by SGIJapan, Ltd.) is used for the calculations. The quantum chemistrycomputation is performed assuming that all of the cations represented bythe structural formulae (α-1) to (α-9) have the most stable structureand are in a vacuum.

In the case where a room-temperature ionic liquid is used for anelectrolyte of a power storage device, the reduction resistance of theroom-temperature ionic liquid results in the degree of electronsreceived from a positive electrode or a negative electrode by a cationincluded in the room-temperature ionic liquid, as described above.

For example, when the LUMO level of a cation is higher than a conductionband of a negative electrode material, a room-temperature ionic liquidincluding the cation is not reduced. The reduction resistance of thecation with respect to lithium can be evaluated by comparing the LUMOlevel of the cation with the LUMO level of a(N-methyl-N-propylpiperidinium) cation having a reduction potentialsubstantially equivalent to an oxidation-reduction potential of lithiumthat is a typical low potential negative electrode material. In otherwords, it can be said that when the LUMO level of a cation in theroom-temperature ionic liquids of embodiments of the present inventionis higher than the LUMO level of a (N-methyl-N-propylpiperidinium)cation, the room-temperature ionic liquids of embodiments of the presentinvention are excellent in reduction resistance.

From Table 1, the LUMO level of the cation represented by the structuralformula (α-1) is −3.047 eV; the LUMO level of the cation represented bythe structural formula (α-2) is −3.174 eV; the LUMO level of the cationrepresented by the structural formula (α-3) is −3.192 eV; the LUMO levelof the cation represented by the structural formula (α-4) is −2.941 eV;the LUMO level of the cation represented by the structural formula (α-5)is −3.013 eV; the LUMO level of the cation represented by the structuralformula (α-6) is −2.877 eV; the LUMO level of the cation represented bythe structural formula (α-7) is −3.125 eV; and the LUMO level of thecation represented by the structural formula (α-8) is −3.102 eV.

The LUMO level of the (N-methyl-N-propylpiperidinium) cation that is acomparative example represented by the structural formula (α-9) is−3.244 eV. The LUMO levels of all of the cations in the room-temperatureionic liquids of embodiments of the present invention are higher than−3.244 eV. Therefore, the room-temperature ionic liquids of embodimentsof the present invention are excellent in the reduction resistance.

That is, reduction resistance of a room-temperature ionic liquid isimproved by an advantageous effect of introducing an electron donatingsubstituent into a molecule.

The oxidation potential of a room-temperature ionic liquid changesdepending on anion species. When any one of (C_(n)F_(2n+1)SO₂)₂N⁻ (n=0to 4), (C_(m)F_(2m+1)SO₃)⁻ (m=0 to 4), and CF₂(CF₂SO₂)₂N⁻ is used as aunivalent anion of the room-temperature ionic liquids of embodiments ofthe present invention, the oxidation potential can be high. When theoxidation potential is high, it means that the oxidation resistance(also referred to as stability against oxidation) is improved. Theoxidation resistance of the room-temperature ionic liquids ofembodiments of the present invention is improved by the interactionbetween a cation in which electric polarization is alleviated because ofan electron donating substituent and the anion described above.

An electrolyte of a power storage device having low reduction potentialand high oxidation potential, that is, a wide oxidation-reductionpotential window can increase the number of materials which can beselected for a positive electrode and a negative electrode and make theelectrolyte stable to the selected positive electrode material andnegative electrode material. Therefore, a power storage device havingexcellent reliability can be realized.

The energy density of a power storage device is caused by a differencebetween an oxidation potential of a positive electrode material and areduction potential of a negative electrode material. Thus, a lowpotential negative electrode material and a high potential positiveelectrode material can be selected by using an electrolyte having widereduction-oxidation potential window. Consequently, a power storagedevice having high energy density can be realized.

In this embodiment, the case where R₁ to R₅ in the general formula (G1)or (G2) are an alkyl group having 1 to 4 carbon atoms is described;however, the number of carbon atoms is not limited to this. The numberof carbon atoms may be 1 to 20. For example, the number of carbon atomscan be greater than or equal to 5. The freezing point can be changed byadjusting the number of carbon atoms. By changing the freezing point, astorage device which can be used in a variety of applications can bemanufactured.

According to this embodiment, a room-temperature ionic liquid havingexcellent reduction resistance and oxidation resistance is used for anelectrolyte of a power storage device; thus, a high-performance powerstorage device having high energy density and excellent reliability canbe obtained.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 2

An electrolyte in a power storage device of one embodiment of thepresent invention includes a nonaqueous solvent and electrolyte salt.The room-temperature ionic liquid of one embodiment of the presentinvention can be used for the nonaqueous solvent in which theelectrolyte salt dissolves. The electrolyte salt dissolved in thenonaqueous solvent may be electrolyte salt including carrier ions suchas an alkali metal ion, an alkaline earth metal ion, a beryllium ion, ora manganese ion. Examples of the alkali metal ion include a lithium ion,a sodium ion, or a potassium ion. Examples of the alkaline earth metalion include a calcium ion, a strontium ion, or a barium ion. Electrolytesalt including a lithium ion is used as the electrolyte salt of thisembodiment. Further, a lithium-ion secondary battery or a lithium-ioncapacitor can be formed by using at least a positive electrode, anegative electrode, and a separator. In this structure, an electricdouble layer capacitor can be obtained by using the room-temperatureionic liquid of one embodiment of the present invention for anelectrolyte, without using the electrolyte salt.

In this embodiment, among the above described power storage devices, alithium-ion secondary battery using an electrolyte which includes aroom-temperature ionic liquid and electrolyte salt including lithium anda manufacturing method of the lithium-ion secondary battery aredescribed with reference to FIGS. 1A and 1B.

A structural example of a lithium-ion secondary battery 130 is shown inFIG. 1A.

The lithium-ion secondary battery 130 in this embodiment includes apositive electrode 148 including a positive electrode current collector142 and a positive electrode active material layer 143, and a negativeelectrode 149 including a negative electrode current collector 101 and anegative electrode active material layer 104. The lithium-ion secondarybattery 130 in FIG. 1A includes a separator 147, a housing 141, and anelectrolyte 146. The separator 147 is provided between the positiveelectrode 148 and the negative electrode 149. The positive electrode148, the negative electrode 149, and the separator 147 are provided inthe housing 141. The electrolyte 146 is included in the housing 141.

For the positive electrode current collector 142, for example, aconductive material can be used. As the conductive material, aluminum(Al), copper (Cu), nickel (Ni), or titanium (Ti) can be used, forexample. In addition, an alloy material containing two or more of theabove-mentioned conductive materials can be used as the positiveelectrode current collector 142. As the alloy material, an Al—Ni alloyor an Al—Cu alloy can be used, for example. Furthermore, a conductivelayer provided by deposition separately on a substrate and thenseparated from the substrate can be also used as the positive electrodecurrent collector 142.

As the positive electrode active material layer 143, a materialcontaining ions serving as carriers and a transition metal can be used,for example. As the material containing ions serving as carriers and atransition metal, a material represented by a general formulaA_(h)M_(i)PO_(j) (h>0, i>0, j>0) can be used, for example. Here, Arepresents, for example, an alkaline metal such as lithium, sodium, orpotassium; or an alkaline earth metal such as calcium, strontium, orbarium; beryllium; or magnesium. M indicates a transition metal such asiron, nickel, manganese, or cobalt. As the material represented by thegeneral formula A_(h)M_(i)PO_(j) (h>0, i>0, j>0), lithium ironphosphate, sodium iron phosphate, or the like can be given. The materialrepresented by A and the material represented by M may be selected fromone or more of each of the above materials.

Alternatively, a material represented by a general formulaA_(h)M_(i)O_(j) (h>0, i>0, j>0) can be used. Here, A represents, forexample, an alkaline metal such as lithium, sodium, or potassium; or analkaline earth metal such as calcium, strontium, or barium; beryllium;or magnesium. M indicates a transition metal such as iron, nickel,manganese, or cobalt. As the material represented by the general formulaA_(h)M_(i)O_(j) (h>0, i>0, j>0), lithium cobaltate, lithium manganate,lithium nickel oxide, or the like can be given. The material representedby A and the material represented by M may be selected from one or moreof each of the above materials.

A material containing lithium is preferably selected for the positiveelectrode active material layer 143 of the lithium-ion secondary batteryin this embodiment. In other words, A in the above general formulaeA_(h)M_(i)PO_(j) (h>0, i>0, j>0) or A_(h)M_(i)O_(j) (h>0, i>0, j>0) ispreferably lithium.

The positive electrode active material layer 143 may be formed byapplying a paste mixed with a conductive additive (for example,acetylene black (AB) or a binder (for example, polyvinylidene fluoride(PVDF))) onto the positive electrode current collector 142, or formed bysputtering. In the case of forming the positive electrode activematerial layer 143 by a coating method, pressure forming may also beemployed, if necessary.

Note that strictly speaking, “active material” refers only to a materialthat relates to insertion and elimination of ions functioning ascarriers. In this specification, however, in the case of using a coatingmethod to form the positive electrode active material layer 143, for thesake of convenience, the positive electrode active material layer 143collectively refers to the material of the positive electrode activematerial layer 143, that is, a substance that is actually a “positiveelectrode active material,” a conductive additive, a binder, etc.

For the negative electrode current collector 101, a simple substance ofcopper (Cu), aluminum (Al), nickel (Ni), or titanium (Ti), or a compoundof any of these elements can be used.

There is no particular limitation on a material used for the negativeelectrode active material layer 104 as long as it can dissolve andprecipitate lithium and can be doped and dedoped with a lithium ion. Forexample, a lithium metal, a carbon based material, silicon, a siliconalloy, or tin can be used. For carbon to/from which a lithium ion can beinserted and extracted, graphite based carbon such as a fine graphitepowder, a graphite fiber, or graphite can be used.

For a nonaqueous solvent of the electrolyte 146, the room-temperatureionic liquids described in Embodiment 1 can be used. For electrolytesalt of the electrolyte 146, electrolyte salt including lithium can beused. Further, the nonaqueous solvent of the electrolyte 146 in whichthe electrolyte salt dissolves is not necessarily a single solvent ofthe room-temperature ionic liquids described in Embodiment 1. Thenonaqueous solvent may be a mixed solvent of plural kinds of solvents inwhich any one of the room-temperature ionic liquids described inEmbodiment 1 and another room-temperature ionic liquid are mixed.

Examples of the electrolyte salt including lithium include lithiumchloride (LiCl), lithium fluoride (LiF), lithium perchlorate (LiClO₄),lithium fluoroborate (LiBF₄), LiAsF₆, LiPF₆, Li(CF₃SO₂)₂N, and the like.The electrolyte salt dissolved in the nonaqueous solvent of theroom-temperature ionic liquids described in Embodiment 1 may beelectrolyte salt which includes a carrier ion and corresponds with thepositive electrode active material layer 143. In this embodiment, theelectrolyte salt including lithium is used as the electrolyte saltbecause lithium is contained in the material used for the positiveelectrode active material layer 143. However, it is preferable to useelectrolyte salt including sodium as the electrolyte salt when amaterial containing sodium is used for the positive electrode activematerial layer 143, for example.

As the separator 147, paper, nonwoven fabric, a glass fiber, a syntheticfiber such as nylon (polyimide), vinylon (a polyvinyl alcohol basedfiber), polyester, acrylic, polyolefin, or polyurethane, or the like maybe used. Note that a material which does not dissolve in the electrolyteshould be selected.

More specific examples of the materials for the separator 147 arehigh-molecular compounds based on fluorine-based polymer, polyether suchas polyethylene oxide and polypropylene oxide, polyolefin such aspolyethylene and polypropylene, polyacrylonitrile, polyvinylidenechloride, polymethyl methacrylate, polymethylacrylate, polyvinylalcohol, polymethacrylonitrile, polyvinyl acetate, polyvinylpyrrolidone,polyethyleneimine, polybutadiene, polystyrene, polyisoprene, andpolyurethane; derivatives thereof; cellulose; paper; and nonwovenfabric, all of which can be used either alone or in a combination.

Next, a lithium-ion secondary battery 131 having a different structurefrom that in FIG. 1A will be described with reference to FIG. 1B. Thelithium-ion secondary battery 131 shown in FIG. 1B includes the positiveelectrode 148 including the positive electrode current collector 142 andthe positive electrode active material layer 143, and the negativeelectrode 149 including the negative electrode current collector 101 andthe negative electrode active material layer 104. In the lithium-ionsecondary battery, a separator 156 is provided between the positiveelectrode 148 and the negative electrode 149 and is impregnated with anelectrolyte.

The components shown in the lithium-ion secondary battery 130 can beused as the negative electrode current collector 101, the negativeelectrode active material layer 104, the positive electrode currentcollector 142, and the positive electrode active material layer 143,which are in FIG. 1B.

The separator 156 is preferably a porous film. As a material of theporous film, a glass fiber, a synthetic resin material, a ceramicmaterial, or the like may be used.

As the electrolyte with which the separator 156 is impregnated, theelectrolyte in the lithium-ion secondary battery 130 can be used.

<Method for Manufacturing Lithium-Ion Secondary Battery>

Here, a method for manufacturing the positive electrode 148 includingthe positive electrode active material layer 143 on the positiveelectrode current collector 142 will be described.

For the material of each of the positive electrode current collector 142and the positive electrode active material layer 143, the abovedescribed materials can be used.

Then, the positive electrode active material layer 143 is formed on thepositive electrode current collector 142. The positive electrode activematerial layer 143 may be formed by a sputtering method or a coatingmethod as described above. In the case of forming the positive electrodeactive material layer 143 by a coating method, the material for thepositive electrode active material layer 143 is mixed with a conductionauxiliary agent, a binder, etc. to form a paste, and the paste isapplied onto the positive electrode current collector 142 and dried toform the positive electrode active material layer 143. In the case offorming the positive electrode active material layer 143 by a coatingmethod, pressure forming may be employed, if necessary. As describedabove, the positive electrode 148 includes the positive electrode activematerial layer 143 formed on the positive electrode current collector142.

Note that as the conduction auxiliary agent, an electron-conductivematerial which does not cause chemical change in the power storagedevice may be used. For example, a carbon material such as graphite orcarbon fibers; a metal material such as copper, nickel, aluminum, orsilver; or a powder or a fiber of a mixture thereof can be used.

Note that as the binder, a polysaccharide, a thermoplastic resin, apolymer with rubber elasticity, or the like such as starch, polyvinylalcohol, carboxymethyl cellulose, hydroxypropyl cellulose, regeneratedcellulose, or diacetyl cellulose, polyvinyl chloride, polyvinylpyrrolidone, polytetrafluoroethylene, polyvinylide fluoride,polyethylene, or polypropylene, ethylene-propylene-diene monomer (EPDM),sulfonated EPDM, styrene-butadiene rubber, butadiene rubber, fluorinerubber, or polyethylene oxide can be given.

Next, a method for manufacturing the negative electrode 149 includingthe negative electrode current collector 101 and the negative electrodeactive material layer 104 will be described.

For the material of each of the negative electrode current collector 101and the negative electrode active material layer 104, the abovedescribed materials can be used.

Next, the negative electrode active material layer 104 is formed on thenegative electrode current collector 101. In this embodiment a lithiumfoil is used. The room-temperature ionic liquid of one embodiment of thepresent invention is excellent in reduction resistance and is stable tolithium which is a negative electrode material having the lowestpotential. Consequently, when the room-temperature ionic liquid is usedfor the electrolyte, the lithium-ion secondary batteries 130 and 131which have high energy density and excellent reliability can beobtained.

In the case where a material other than a lithium foil is used for thenegative electrode active material layer 104, the negative electrodeactive material layer 104 can be manufactured in a manner similar tothat of the positive electrode active material layer 143. For example,when silicon is used as the negative electrode active material layer104, a material obtained by depositing microcrystalline silicon and thenremoving amorphous silicon from the microcrystalline silicon by etchingmay be used. When amorphous silicon is removed from microcrystallinesilicon, the surface area of the remaining microcrystalline silicon isincreased. A chemical vapor deposition method or a physical vapordeposition method can be used as the deposition method of themicrocrystalline silicon. For example, a plasma CVD method can be usedas the chemical vapor deposition method and a sputtering method can beused as the physical vapor deposition method. Note that the abovedescribed conduction auxiliary agent and the binder can also be used.

The electrolyte 146 and the electrolyte with which the separator 156 isimpregnated can be manufactured by mixing electrolyte salt including acarrier ion and any one of the room-temperature ionic liquids describedin Embodiment 1. In this embodiment, Li(CF₃SO₂)₂N is used as theelectrolyte salt including lithium.

A variety of reactions can be applied to a method for synthesizing theroom-temperature ionic liquids described in Embodiment 1. As an example,Synthesis Scheme (S-1) can be employed.

In the above Scheme (S-1), the reaction from a general formula (α-10) toa general formula (α-11) is alkylation of amine by an amine compound anda carbonyl compound in the presence of hydrido. For example, excessiveformic acid can be used as the hydrido source. CH₂O is used as thecarbonyl compound in the room-temperature ionic liquid of one embodimentof the present invention.

In the above Scheme (S-1), the reaction from the general formula (α-11)to a general formula (α-12) is alkylation by a tertiary amine compoundand an alkyl halide compound, which synthesizes quaternary ammoniumsalt. Propane halide is used as the alkyl halide compound in theroom-temperature ionic liquid of one embodiment of the presentinvention. X is halogen, preferably bromine or iodine, which has highreactivity, more preferably iodine.

Through ion exchange between the quaternary ammonium salt represented bythe general formula (α-12) and desired metal salt, the room-temperatureionic liquid of one embodiment of the present invention can be obtained.Lithium metal salt can be used in the Synthesis Scheme (S-1).

Next, a top view of a specific structure of the lithium-ion secondarybattery 130 in FIG. 1A which is laminated is shown in FIG. 2A. Aperspective view of a specific structure of the lithium-ion secondarybattery 131 in FIG. 1B which is a button type is shown in FIG. 2B. Amethod for assembling the button-type lithium-ion secondary battery 131in FIG. 2B is shown in FIGS. 3A and 3B and FIG. 4.

The laminated lithium-ion secondary battery 130 in FIG. 2A includes thepositive electrode 148 including the positive electrode currentcollector 142 and the positive electrode active material layer 143, andthe negative electrode 149 including the negative electrode currentcollector 101 and the negative electrode active material layer 104,which are described above. The laminated lithium-ion secondary battery130 in FIG. 2A includes the separator 147 between the positive electrode148 and the negative electrode 149. In the lithium-ion secondary battery130, the positive electrode 148, the negative electrode 149, and theseparator 147 are placed in the housing 141 and the electrolyte 146 isincluded in the housing 141.

In FIG. 2A, the negative electrode current collector 101, the negativeelectrode active material layer 104, the separator 147, the positiveelectrode active material layer 143, and the positive electrode currentcollector 142 are arranged in this order from the bottom side. Thenegative electrode current collector 101, the negative electrode activematerial layer 104, the separator 147, the positive electrode activematerial layer 143, and the positive electrode current collector 142 areprovided in the housing 141. The housing 141 is filled with theelectrolyte 146.

The positive electrode current collector 142 and the negative electrodecurrent collector 101 in FIG. 2A also function as terminals forelectrical contact with the outside. For this reason, part of each ofthe positive electrode current collector 142 and the negative electrodecurrent collector 101 is arranged outside the housing 141 so as to beexposed.

Note that FIG. 2A shows one example of the laminated lithium-ionsecondary battery 130 and the laminated lithium-ion secondary battery130 may have other structures.

The button-type lithium-ion secondary battery 131 in FIG. 2B includesthe separator 156 provided between the positive electrode 148 and thenegative electrode 149. The separator 156 is impregnated with anelectrolyte. The specific structure and the assembling method of thebutton-type lithium-ion secondary battery 131 in FIG. 2B will bedescribed with reference to FIGS. 3A and 3B and FIG. 4.

First, a first housing 171 is prepared. A bottom surface of the firsthousing 171 is a circle and the side of the first housing 171 is arectangle. That is, the first housing 171 is a dish having a columnarshape. It is necessary to use a conductive material for the firsthousing 171 in order that the positive electrode 148 can be electricallyconnected to the outside. For example, the first housing 171 may beformed of a metal material. The positive electrode 148 including thepositive electrode current collector 142 and the positive electrodeactive material layer 143 is provided in the first housing 171 (see FIG.3A).

In addition, a second housing 172 is prepared. A bottom surface of thesecond housing 172 is a circle and the side of the second housing 172 isa trapezoid in which an upper base is longer than a lower base. That is,the second housing 172 is a dish having a columnar shape. The diameterof the dish is smallest at the bottom and increases upward. Note thatthe diameter of the second housing 172 is smaller than the diameter ofthe bottom surface of the first housing 171. The reason is describedlater.

It is necessary to use a conductive material for the second housing 172in order that the negative electrode 149 can be electrically connectedto the outside. For example, the second housing 172 may be formed of ametal material. The negative electrode 149 including the negativeelectrode current collector 101 and the negative electrode activematerial layer 104 is provided in the second housing 172.

A ring-shaped insulator 173 is provided so as to surround the side ofthe positive electrode 148 provided in the first housing 171. Thering-shaped insulator 173 has a function of insulating the negativeelectrode 149 and the positive electrode 148 from each other. Thering-shaped insulator 173 is preferably formed using an insulatingresin.

The second housing 172 in which the negative electrode 149 is providedshown in FIG. 3B is installed in the first housing 171 in which thering-shaped insulator 173 is provided, with the separator 156 which isalready impregnated with the electrolyte interposed therebetween. Thesecond housing 172 can be fit in the first housing 171 because thediameter of the second housing 172 is smaller than the diameter of thebottom surface of the first housing 171 (see FIG. 4).

As described above, the positive electrode 148 and the negativeelectrode 149 are insulated from each other by the ring-shaped insulator173, so that the positive electrode 148 and the negative electrode 149do not short-circuit.

Note that one example of the button-type lithium-ion secondary battery131 is shown in FIG. 2B and the button-type lithium-ion secondarybattery 131 may have other structures.

In FIG. 2A, an example of the lithium-ion secondary battery 130 in FIG.1A which is laminated is shown, and in FIGS. 3A and 3B and FIG. 4, anexample of the lithium-ion secondary battery 131 in FIG. 2B which isbutton type is shown; however, structures of the lithium-ion secondarybatteries 130 and 131 are not limited to these. The lithium-ionsecondary batteries 130 and 131 shown in FIGS. 1A and 1B may havevarious structures such as a button type, a stack type, a cylinder type,and a laminate type.

As described above, according to this embodiment, a room-temperatureionic liquid having excellent reduction resistance and oxidationresistance is used for an electrolyte of a power storage device; thus, ahigh-performance power storage device having high energy density andexcellent reliability can be obtained.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 3

In this embodiment, an application mode of the power storage devicedescribed in Embodiment 2 will be described with reference to FIG. 5.

The power storage device described in Embodiment 2 can be used inelectronic appliances, e.g., cameras such as digital cameras or videocameras, digital photo frames, mobile phones (also referred to ascellular phones or cellular phone devices), portable game machines,portable information terminals, and audio reproducing devices. Further,the power storage device can be used in electric propulsion vehiclessuch as electric vehicles, hybrid electric vehicles, train vehicles,maintenance vehicles, carts, and wheelchairs. Here, as a typical exampleof the electric propulsion vehicles, a wheelchair is described.

FIG. 5 is a perspective view of an electric wheelchair 501. The electricwheelchair 501 includes a seat 503 where a user sits down, a backrest505 provided behind the seat 503, a footrest 507 provided at the frontof and below the seat 503, armrests 509 provided on the left and rightof the seat 503, and a handle 511 provided above and behind the backrest505. A controller 513 for controlling the operation of the wheelchair isprovided for one of the armrests 509. A pair of front wheels 517 isprovided at the front of and below the seat 503 through a frame 515provided below the seat 503. A pair of rear wheels 519 is providedbehind and below the seat 503. The rear wheels 519 are connected to adriving portion 521 including a motor, a brake, a gear, and the like. Acontrol portion 523 including a battery, a power controller, a controlmeans, and the like is provided under the seat 503. The control portion523 is connected to the controller 513 and the driving portion 521. Thedriving portion 521 is driven through the control portion 523 with theoperation of the controller 513 by the user. The control portion 523controls the operation of moving forward, moving back, turning around,and the like, and the speed of the electric wheelchair 501.

The power storage device described in Embodiment 2 can be used in thebattery of the control portion 523. The battery of the control portion523 can be externally charged by electric power supply using plug-insystems or contactless power feeding. Note that in the case where theelectric propulsion vehicle is a train vehicle, the train vehicle can becharged by electric power supply from an overhead cable or a conductorrail.

Example 1

In this example, an example of a method for producing1,2-dimethyl-1-propylpiperidinium bis(trifluoromethanesulfonyl)imide(abbreviation: 2 mPP13-TFSA) represented by a structural formula (α-13)will be described.

2-Methylpiperidine (1.98 g, 200 mmol) was gradually added to formic acid(15.6 g, 300 mmol) while cooling with water. Next, formaldehyde (22.5ml, 300 mmol) was added to this solution. This solution was heated to100° C. and cooled back to room temperature after a bubble generation,and was stirred for about 30 minutes. Then, the solution was heated andrefluxed for one hour.

The formic acid was neutralized with sodium carbonate, the solution wasextracted with hexane and dried over magnesium sulfate, and the solventwas distilled off, whereby 1,2-dimethylpiperidine (12.82 g, 113 mmol)which was light yellow liquid was obtained.

Bromopropane (20.85 g, 170 mmol) was added to methylene chloride (10 ml)to which the obtained light yellow liquid was added, and was heated andrefluxed for 24 hours, so that a white precipitate was generated. Afterfiltration, the remaining substance was recrystallized from ethanol andethyl acetate and dried under reduced pressure at 80° C. for 24 hours,whereby 1,2-dimethyl-1-propylpiperidinium bromide (11.93 g, 48 mmol)which was a white solid was obtained.

1,2-Dimethyl-1-propylpiperidinium bromide (5.3 g, 22 mmol) and lithiumbis(trifluoromethanesulfonyl)imide (7.09 g, 25 mmol) were mixed andstirred in pure water, so that a room-temperature ionic liquid which isinsoluble in water was obtained immediately. The obtainedroom-temperature ionic liquid was extracted with methylene chloride andthen washed with pure water six times and dried in vacuum at 100° C.;thus, 1,2-dimethyl-1-propylpiperidiniumbis(trifluoromethanesulfonyl)imide (9.37 g, 21 mmol) was obtained.

The compound obtained through the above steps was identified as1,2-dimethyl-1-propylpiperidinium bis(trifluoromethanesulfonyl)imidewhich is a target substance by using a nuclear magnetic resonance (NMR)method and mass spectrometry.

¹H NMR data of the obtained compound is shown below.

¹H-NMR (CDCl₃, 400 MHz, 298 K): δ (ppm): 1.00, 1.03, 1.06 (t, 3H), 1.29,1.34, 1.40 (d, 3H), 1.59-1.88 (m, 8H), 2.85, 2.90, 3.00, 3.07 (s, 3H),2.85-2.98, 3.20-3.42 (m, 2H), 3.20-3.54 (m, 2H), 3.50-3.54 (m, 1H)

In addition, FIGS. 6A and 6B show ¹H NMR charts. Note that FIG. 6B is achart showing an enlargement of FIG. 6A in the range of 0.750 ppm to3.75 ppm.

The measurement results of the electro spray ionization mass spectrum(ESI-MS) of the obtained compound are shown below.

MS (ESI-MS): m/z=156.2 (M)⁺; C₁₀H₂₂N (156.2), 279.98 (M)⁻; C₂F₆NO₄S₂(280.15)

Example 2

Next, an example of a method for producing1,3-dimethyl-1-propylpiperidinium bis(trifluoromethanesulfonyl)imide(abbreviation: 3 mPP13-TFSA) represented by a structural formula (α-14)will be described.

3-Methylpiperidine (1.98 g, 200 mmol) was gradually added to formic acid(15.6 g, 300 mmol) while cooling with water. Next, formaldehyde (22.5ml, 300 mmol) was added to this solution. This solution was heated to100° C. and brought back to room temperature after a bubble generation,and was stirred for about 30 minutes. Then, the solution was heated andrefluxed for one hour.

The formic acid was neutralized with sodium carbonate, the solution wasextracted with hexane and dried over magnesium sulfate, and the solventwas distilled off, whereby 1,3-dimethylpiperidine (12.82 g, 113 mmol)which was light yellow liquid was obtained.

Bromopropane (20.85 g, 170 mmol) was added to methylene chloride (10 ml)to which this light yellow liquid was added, and was heated and refluxedfor 24 hours, so that a white precipitate was generated. Afterfiltration, the remaining substance was recrystallized from ethanol andethyl acetate and dried under reduced pressure at 80° C. for 24 hours,whereby 1,3-dimethyl-1-propylpiperidinium bromide (19.42 g, 82 mmol)which is a white solid was obtained.

1,3-Dimethyl-1-propylpiperidinium bromide (10.60 g, 44 mmol) and lithiumbis(trifluoromethanesulfonyl)imide (14.18 g, 50 mmol) were mixed andstirred to be in equimolar amounts in pure water, so that aroom-temperature ionic liquid which is insoluble in water was obtainedimmediately.

The room-temperature ionic liquid was extracted with methylene chlorideand then washed with pure water six times and dried in vacuum at 100°C.; thus, 1,3-dimethyl-1-propylpiperidiniumbis(trifluoromethanesulfonyl)imide (18.31 g, 42 mmol) was obtained.

The compound obtained through the above steps was identified as1,3-dimethyl-1-propylpiperidinium bis(trifluoromethanesulfonyl)imidewhich is a target substance by using a nuclear magnetic resonance (NMR)method and mass spectrometry.

¹H NMR data of the obtained compound is shown below.

¹H-NMR (CDCl₃, 400 MHz, 298 K): δ (ppm) 0.93-1.06 (m, 6H), 1.13-1.23,1.75-1.95 (m, 2H), 1.60-1.95 (m, 2H), 1.75-1.95 (m, 2H), 1.95-2.12 (m,1H), 2.72-2.84, 3.30-3.42 (m, 2H), 2.98, 3.01, 3.02, 3.07 (s, 3H),3.07-3.52 (m, 2H), 3.19-3.28 (m, 2H)

In addition, FIGS. 7A and 7B show ¹H NMR charts. Note that FIG. 7B is achart showing an enlargement of FIG. 7A in the range of 0.750 ppm to3.75 ppm.

The measurement results of the electro spray ionization mass spectrum(ESI-MS) of the obtained compound are shown below.

MS (ESI-MS): m/z=156.2 (M)⁺; C₁₀H₂₂N (156.2), 279.98 (M)⁻; C₂F₆NO₄S₂(280.15)

Example 3

Linear sweep voltammograms of 2 mPP13-TFSA and 3 mPP13-TFSA which areshown in the above Examples were measured and potential windows of theabove room-temperature ionic liquid were calculated.N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imideproduced by KANTO CHEMICAL CO., INC. was used as a comparative sample.

The measurement was performed by using electrochemical measurementsystem HZ-5000 produced by HOKUTO DENKO CORPORATION in a glove box withan argon atmosphere. A glassy carbon electrode was used as a workingelectrode and a platinum wire was used for an opposite electrode. Asilver wire immersed in a solution in which silvertrifluoromethanesulfonate was dissolved in 1-ethyl-3-methylimidazoliumbis(trifluoromethanesulfonyl)imide at a concentration of 0.1 M was usedfor a reference electrode. Oxidation-reduction potential of theroom-temperature ionic liquid was corrected with reference to theoxidation-reduction potential of ferrocene (Fc/Fc⁺).

A linear sweep voltammogram of each of 2 mPP13-TFSA, 3 mPP13-TFSA, andthe comparative sample is shown in FIG. 8. A “potential window” in thisexample indicates a difference between an oxidation potential and areduction potential. In FIG. 8, a potential at which an electric currentdensity of −1 mA/cm² was detected during the scanning of potentials wascalculated as a reduction potential. Further, in FIG. 8, a potential atwhich an electric current density of 1 mA/cm² was detected during thescanning of the potentials was calculated as an oxidation potential. Thepotential window was calculated by subtracting a “reduction potential”from an “oxidation potential”.

In FIG. 8, a thin line denotes the comparative sample and thick linesdenote 2 mPP13-TFSA and 3 mPP13-TFSA which are embodiments of thepresent invention. From FIG. 8, the reduction potential of thecomparative sample is −3.4 eV, the oxidation potential thereof is 2.5eV, and the potential window thereof is 5.9 eV. The reduction potentialof 2 mPP13-TFSA is −3.6 eV, the oxidation potential thereof is 2.7 eV,and the potential window thereof is 6.3 eV. The reduction potential of 3mPP13-TFSA is −3.6 eV, the oxidation potential thereof is 2.7 eV, andthe potential window thereof is 6.3 eV.

It was confirmed that 2 mPP13-TFSA and 3 mPP13-TFSA which are oneembodiment of the present invention each have a lower reductionpotential and a higher oxidation potential than those of the comparativesample. Higher reduction resistance was confirmed compared to thecomparative sample. That is, stability against a low potential negativeelectrode of a lithium metal, silicon, tin, or the like was improved byintroduction of an electron donating substituent. Further, 2 mPP13-TFSAand 3 mPP13-TFSA which are one embodiment of the present invention eachhave a higher oxidation potential than that of the comparative sample,whereby the oxidation resistance of 2 mPP13-TFSA and 3 mPP13-TFSA isexcellent. Consequently, 2 mPP13-TFSA and 3 mPP13-TFSA each have a widepotential window. As described above, a low potential negative electrodematerial and a high potential positive electrode material can beselected by using the room-temperature ionic liquid of one embodiment ofthe present invention for an electrolyte; thus, a power storage devicehaving high energy density can be obtained.

Example 4

Next, an example of a method for producing1,3-dimethyl-1-propylpiperidinium bis(fluorosulfonyl)imide(abbreviation: 3 mPP13-FSA) represented by a structural formula (α-15)will be described.

Since steps for obtaining 1,3-dimethyl-1-propylpiperidinium bromide arethe same as the steps described in Example 2, the description is omittedhere. 1,3-dimethyl-1-propylpiperidinium bromide (17.02 g, 72 mmol)obtained in a manner similar to that in Example 2 and potassiumbis(fluorosulfonyl)imide (17.04 g, 78 mmol) were mixed and stirred inpure water, so that a room-temperature ionic liquid which is insolublein water was obtained immediately.

The room-temperature ionic liquid was extracted with methylene chlorideand then washed with pure water six times and dried in vacuum at roomtemperature through a trap at −80° C.; thus,1,3-dimethyl-1-propylpiperidinium bis(fluorosulfonyl)imide (20.62 g, 61mmol) which is a room-temperature ionic liquid was obtained.

The compound obtained through the above steps was identified as1,3-dimethyl-1-propylpiperidinium bis(fluorosulfonyl)imide which is atarget substance by using a nuclear magnetic resonance (NMR) method andmass spectrometry.

¹H NMR data of the obtained compound is shown below.

¹H-NMR (CDCl₃, 400 MHz, 298 K): δ (ppm) 1.02-1.09 (m, 6H), 1.21-1.26,1.69-1.75 (m, 2H), 1.83-1.91 (m, 2H), 1.94-1.97 (m, 2H), 1.97-2.15 (m,1H), 2.77-2.87, 3.30-3.43 (m, 2H), 3.05, 3.10 (s, 3H), 3.15-3.54 (m,2H), 3.25-3.29 (m, 2H)

Further, ¹H-NMR charts are shown in FIGS. 9A and 9B. Note that FIG. 9Bis a chart showing an enlargement of FIG. 9A in the range of 0.750 ppmto 3.75 ppm.

The measurement results of the electro spray ionization mass spectrum

(ESI-MS) of the obtained compound are shown below.

MS (ESI-MS): m/z=156.2 (M)⁺; C₁₀H₂₂N (156.2), 179.98 (M)⁻; F₂NO₄S₂(180.13)

Example 5

Next, results of charge and discharge characteristics of a coin-typelithium-ion secondary battery cell using 3 mPP13-FSA in Example 4 for anonaqueous electrolyte are shown. Note that in this example, a Sample Ais the coin-type lithium-ion secondary battery cell using 3 mPP13-FSAfor a nonaqueous electrolyte.

First, a method for manufacturing the Sample A will be described withreference to FIG. 10. As a nonaqueous electrolyte of the Sample A, 1.84g (6.4 mmol) of lithium bis(trifluoromethanesulfonyl)imide(abbreviation: LiTFSA) and 6.81 g (20.0 mmol) of 3 mPP13-FSA were mixedin a glove box with an argon atmosphere.

Commercially available products were used for the positive electrode148, the negative electrode 149, the ring-shaped insulator 173, and theseparator 156, other than the nonaqueous electrolyte, which were in theSample A. Specifically, an electrode manufactured by Piotrek Co., Ltd.was used as the positive electrode 148. The positive electrode activematerial layer 143 was formed of lithium cobaltate and the positiveelectrode current collector 142 was formed of an aluminum foil. Thecapacitance per weight of the electrode used as the positive electrode148 is 112 mAh/g. The negative electrode active material layer 104 inthe negative electrode 149 was formed of a lithium foil. For theseparator 156, GF/C which is a glass fiber filter produced by WhatmanLtd. was used. Then, the positive electrode 148, the negative electrode149, and the separator 156 were impregnated with the nonaqueouselectrolyte. Commercially available objects were used for the housings171 and 172. The housing 171 electrically connects the positiveelectrode 148 to the outside and the housing 172 electrically connectsthe negative electrode 149 to the outside. The housings 171 and 172 wereformed of stainless steel (SUS). In addition, a spacer 181 and a washer183 formed of stainless steel (SUS) were prepared; commerciallyavailable objects were used as the spacer 181 and the washer 183.

As shown in FIG. 10, the housing 171, the positive electrode 148, theseparator 156, the ring-shaped insulator 173, the negative electrode149, the spacer 181, the washer 183, and the housing 172 were stacked inthis order from the bottom side. The positive electrode 148, thenegative electrode 149 and the separator 156 were impregnated with thenonaqueous electrolyte. Then, the housing 171 and the housing 172 werecrimped to each other with a “coin cell crimper”. Thus, the Sample A wasmanufactured.

Next, a coin-type lithium-ion secondary battery cell for comparison(Sample B) was manufactured in a manner similar to that of the Sample A(see FIG. 10). Note that the difference between the Sample A and theSample B is components of nonaqueous electrolytes. For a nonaqueouselectrolyte of the Sample B, LiTFSA and N-methyl-N-propylpiperidiniumbis(fluoromethanesulfonyl)imide (abbreviation: PP13-FSA) were used.Specifically, 2.82 g (9.8 mmol) of LiTFSA and 10.02 g (31.0 mmol) ofPP13-FSA were mixed in a glove box with an argon atmosphere.

The charge and discharge characteristics of each of the Sample A and theSample B were measured. The charge and discharge characteristics weremeasured by using a charge-discharge measuring device (produced by TOYOSYSTEM Co., LTD). For the measurements of charge and discharge, aconstant current mode was used. The charge and discharge were performedwith a current of 0.15 mA at a rate of 0.1 C. The upper limit voltagewas 4.2 V and the lower limit voltage was 2.5 V. Note that charging anddischarging were in one cycle. In this embodiment, 50 cycles wereperformed. The samples were measured at room temperature.

The cycle characteristics of each of the Samples A and B which weremeasured are shown in FIG. 11. The horizontal axis of FIG. 11 indicatesthe number of cycles. The vertical axis of FIG. 11 indicates a capacitymaintenance rate of the coin-type lithium-ion secondary batteries. Notethat the capacity maintenance rate refers to a percentage of acapacitance after certain cycles to the highest capacitance during 50cycles. A solid line in FIG. 11 shows the charge and dischargecharacteristics of the Sample A. A dotted line in FIG. 11 is the chargeand discharge characteristics of the Sample B. From FIG. 11, it wasconfirmed that the Sample A is less likely to deteriorate because thecapacity maintenance rate after 50 cycles is high.

As described above, when a room-temperature ionic liquid havingexcellent reduction resistance is used for a nonaqueous electrolyte, ahigh-performance power storage device having excellent charge anddischarge characteristics can be obtained.

This application is based on Japanese Patent Application serial no.2010-149169 filed with Japan Patent Office on Jun. 30, 2010, the entirecontents of which are hereby incorporated by reference.

1. A power storage device comprising a room-temperature ionic liquidrepresented by a general formula (G1), wherein one of R₁ to R₅represents any one of an alkyl group having 1 to 20 carbon atoms, amethoxy group, a methoxymethyl group, and a methoxyethyl group; theother four of R₁ to R₅ represent hydrogen atoms; and A⁻ represents aunivalent imide anion, a univalent methide anion, a perfluoroalkylsulfonic acid anion, tetrafluoroborate, or hexafluorophosphate.


2. The power storage device according to claim 1, wherein the number ofthe alkyl group is 1 to
 4. 3. The power storage device according toclaim 1, wherein the A⁻ in the room-temperature ionic liquid is any oneof univalent anion selected from (C_(n)F_(2n+1)SO₂)₂N⁻ (n=0 to 4),(C_(m)F_(2m+1)SO₃)⁻ (m=0 to 4), and CF₂ (CF₂SO₂)₂N⁻.
 4. A lithium-ionsecondary battery comprising: a positive electrode, a negativeelectrode, a separator, and an electrolyte salt, wherein the lithium-ionsecondary battery is the power storage device according to claim 1, andwherein the electrolyte salt includes a lithium ion.
 5. An electricdouble layer capacitor comprising: a positive electrode, a negativeelectrode, and a separator, wherein the electric double layer capacitoris the power storage device according to claim
 1. 6. A lithium-ioncapacitor comprising, a positive electrode, a negative electrode, aseparator, and an electrolyte salt, wherein the lithium-ion capacitor isthe power storage device according to claim 1, and wherein theelectrolyte salt includes a lithium ion.
 7. A power storage devicecomprising a room-temperature ionic liquid represented by a generalformula (G1), wherein one of two of R₁ to R₅ represents any one of analkyl group having 1 to 20 carbon atoms, a methoxy group, amethoxymethyl group, and a methoxyethyl group; wherein the other of twoof R₁ to R₅ represents any one of an alkyl group having 1 to 20 carbonatoms, a methoxy group, a methoxymethyl group, and a methoxyethyl group;the other three of R₁ to R₅ represent hydrogen atoms; and A⁻ representsa univalent imide anion, a univalent methide anion, a perfluoroalkylsulfonic acid anion, tetrafluoroborate, or hexafluorophosphate.


8. The power storage device according to claim 7, wherein the number ofthe alkyl group is 1 to
 4. 9. The power storage device according toclaim 7, wherein the A⁻ in the room-temperature ionic liquid is any oneof univalent anion selected from (C_(n)F_(2n+1)SO₂)₂N⁻ (n=0 to 4),(C_(m)F_(2m+1)SO₃)⁻ (m=0 to 4), and CF₂(CF₂SO₂)₂N⁻.
 10. A lithium-ionsecondary battery comprising: a positive electrode, a negativeelectrode, a separator, and an electrolyte salt, wherein the lithium-ionsecondary battery is the power storage device according to claim 7, andwherein the electrolyte salt includes a lithium ion.
 11. An electricdouble layer capacitor comprising: a positive electrode, a negativeelectrode, and a separator, wherein the electric double layer capacitoris the power storage device according to claim
 7. 12. A lithium-ioncapacitor comprising, a positive electrode, a negative electrode, aseparator, and an electrolyte salt, wherein the lithium-ion capacitor isthe power storage device according to claim 7, and wherein theelectrolyte salt includes a lithium ion.
 13. A power storage devicecomprising a room-temperature ionic liquid represented by a generalformula (G2), wherein one of R₁ and R₂ represents any one of an alkylgroup having 1 to 20 carbon atoms, a methoxy group, a methoxymethylgroup, and a methoxyethyl group; the other of R₁ and R₂ represents ahydrogen atom; and A⁻ represents a univalent imide anion, a univalentmethide anion, a perfluoroalkyl sulfonic acid anion, tetrafluoroborate,or hexafluorophosphate.


14. The power storage device according to claim 13, wherein the numberof the alkyl group is 1 to
 4. 15. The power storage device according toclaim 13, wherein the A⁻ in the room-temperature ionic liquid is any oneof univalent anion selected from (C_(n)F_(2n+1)SO₂)₂N⁻ (n=0 to 4),(C_(m)F_(2m+1)SO₃)⁻ (m=0 to 4), and CF₂(CF₂SO₂)₂N⁻.
 16. A lithium-ionsecondary battery comprising: a positive electrode, a negativeelectrode, a separator, and an electrolyte salt, wherein the lithium-ionsecondary battery is the power storage device according to claim 13, andwherein the electrolyte salt includes a lithium ion.
 17. An electricdouble layer capacitor comprising: a positive electrode, a negativeelectrode, and a separator, wherein the electric double layer capacitoris the power storage device according to claim
 13. 18. A lithium-ioncapacitor comprising, a positive electrode, a negative electrode, aseparator, and an electrolyte salt, wherein the lithium-ion capacitor isthe power storage device according to claim 13, and wherein theelectrolyte salt includes a lithium ion.
 19. A power storage devicecomprising a room-temperature ionic liquid represented by a generalformula (G3), wherein A⁻ represents a univalent imide anion, a univalentmethide anion, a perfluoroalkyl sulfonic acid anion, tetrafluoroborate,or hexafluorophosphate.


20. The power storage device according to claim 19, wherein the A⁻ inthe room-temperature ionic liquid is any one of univalent anion selectedfrom (C_(n)F_(2n+1)SO₂)₂N⁻ (n=0 to 4), (C_(m)F_(2m+1)SO₃)⁻ (m=0 to 4),and CF₂(CF₂SO₂)₂N⁻.
 21. A lithium-ion secondary battery comprising: apositive electrode, a negative electrode, a separator, and anelectrolyte salt, wherein the lithium-ion secondary battery is the powerstorage device according to claim 19, and wherein the electrolyte saltincludes a lithium ion.
 22. An electric double layer capacitorcomprising: a positive electrode, a negative electrode, and a separator,wherein the electric double layer capacitor is the power storage deviceaccording to claim
 19. 23. A lithium-ion capacitor comprising, apositive electrode, a negative electrode, a separator, and anelectrolyte salt, wherein the lithium-ion capacitor is the power storagedevice according to claim 19, and wherein the electrolyte salt includesa lithium ion.
 24. A power storage device comprising a room-temperatureionic liquid represented by a general formula (G4), wherein A⁻represents a univalent imide anion, a univalent methide anion, aperfluoroalkyl sulfonic acid anion, tetrafluoroborate, orhexafluorophosphate.


25. The power storage device according to claim 24, wherein the A⁻ inthe room-temperature ionic liquid is any one of univalent anion selectedfrom (C_(n)F_(2n+1)SO₂)₂N⁻ (n=0 to 4), (C_(m)F_(2m+1)SO₃)⁻ (m=0 to 4),and CF₂(CF₂SO₂)₂N⁻.
 26. A lithium-ion secondary battery comprising: apositive electrode, a negative electrode, a separator, and anelectrolyte salt, wherein the lithium-ion secondary battery is the powerstorage device according to claim 24, and wherein the electrolyte saltincludes a lithium ion.
 27. An electric double layer capacitorcomprising: a positive electrode, a negative electrode, and a separator,wherein the electric double layer capacitor is the power storage deviceaccording to claim
 24. 28. A lithium-ion capacitor comprising: apositive electrode, a negative electrode, a separator, and anelectrolyte salt, wherein the lithium-ion capacitor is the power storagedevice according to claim 24, and wherein the electrolyte salt includesa lithium ion.